A quarter of all police shootings involve unarmed suspects. In a few recent cases, officers mistook cell phones and hairbrushes for guns, and shot and killed the victims. Now a study may explain—in part—these errors. Researchers found that when a person holds a gun, they’re more likely to think they see a weapon being carried by another. That study is in the Journal of Experimental Psychology: Human Perception and Performance. [Jessica K. Witt & James R. Brockmole, "Action Alters Object Identification: Wielding a Gun Increases the Bias to See Guns," link to come]

Researchers had volunteers hold a Wii handgun or a foam ball. Then they flashed images of people either holding guns or objects like soda cans, and asked volunteers to decide if they'd seen a weapon. The subjects holding the Wii gun were more likely to mistakenly see a gun in the hands of their onscreen "foe." When researchers varied the experiment, subjects holding shoes were more likely to see shoes onscreen.

The reason, the authors say, is that just planning to possibly use an object—like a pistol—might prime the brain for spotting that object. Which might be a great advantage for quickly noting when a suspect is indeed armed. But can cause tragic overreactions when there’s really no gun in sight.